1. Field of the Invention
The invention relates to a laser plate making apparatus for forming a press plate useful for, for example, a gravure printing press.
2. Background of the Invention
In many conventional laser plate making apparatus, a laser light source having a laser head is used to direct a laser beam onto a form plate sheet. The form plate sheet is wound around a plate cylinder which rotates in a main scanning direction. In addition, the laser light source is moved in the axial or sub scanning direction of the plate cylinder such that recesses are formed over substantially the entire surface of the form plate sheet to form a press plate. In this case, the recesses on the press plate form a halftone dot image.
FIG. 8 schematically shows a typical structure of a laser optical system employed in a laser plate making apparatus based on conventional technology. In FIG. 8, a laser head 1 is shown which includes a laser diode 2, a collimator lens 3 and an objective lens 4.
In FIG. 8, reference numeral 5 represents a beam profile of a laser beam emitted from the laser diode 2, and a luminous stripe width in the horizontal direction of the laser beam 5 is represented by d.sub.LDH. Further, reference numeral 6 represents a focal position, or a beam profile of the laser beam on the form plate sheet, and the focal stripe width in the horizontal direction of the laser beam 6 is represented by d.sub.H.
In the laser optical system of the example in FIG. 8, the laser beam 5 having a luminance stripe width d.sub.LDH is emitted from the laser diode 2 having a light emitting power of about 1 W and is collimated by the collimator lens 3. The laser beam 5 is then collected through the objective lens 4, so that it forms an image as a focal stripe width d.sub.H at the focal position of the objective lens 4. Here, the focal length of the collimator lens 3 is represented by f.sub.1 and the focal length of the objective lens 4 is represented by f.sub.2.
Typical values for the laser optical system shown in FIG. 8, are as follows:
luminance stripe width d.sub.LDH =200 .mu.m PA1 focal length f.sub.1 of the collimator lens 3=11.67 mm PA1 focal length f.sub.2 of the objective lens 4=7 mm PA1 focal stripe width d.sub.H =120 .mu.m.
In this case, the focal stripe distance d.sub.H is determined by the relation of the following Equation 1. EQU d.sub.H =(f.sub.2 /f.sub.1).multidot.d.sub.LDH Equation 1
In a laser optical system employed in a laser plate making apparatus for making a press plate, the position of the image surface frequently changes due to factors such as eccentricity of the plate cylinder, variations in the thickness of the form plate and others. As such, it is desirable to have a relatively deep focal depth in such laser optical systems in order to compensate for eccentricity and the like, that is, where there is a wide allowable range of positions for the image surface in which an image can be clearly formed.
Here, the focal depth of the laser plate making apparatus will be considered.
As shown in FIG. 9, a numerical aperture (NA) of a lens LZ can be expressed as NA=N sin .theta..sub.2 (N represents a refractive index of the lens LZ) when an opening angle of a bundle of rays L.sub.1 is represented by .theta..sub.2. In a case of an optical pickup for reading pits of a compact disc or the like, since the laser beam is utilized under the condition that a part of the bundle of rays of the laser beam is blocked by the lens LZ, or it is so-called eclipsed, the NA of the lens LZ directly relates to the focal depth.
However, in the laser plate making apparatus, since the efficiency of the optical power is important, the optical system is configured such that the laser beam as the flux of light in an optical system of the lens LZ or the like is not eclipsed. Therefore, in the laser plate making apparatus, the optical combination efficiency is increased. Accordingly, in the laser plate making apparatus, the width D.sub.1 of the laser beam incident on the lens LZ is smaller than the effective diameter D.sub.2 of the lens LZ, and thus an effective NA (hereinafter referred to as an effective NA) becomes different from the NA of the lens LZ. The effective NA is expressed as Equation 2. EQU Effective NA=N sin .theta..sub.1 Equation 2
(N represents a refractive index of the lens LZ)
Here, consideration is made on how much the size of the beam spot will be changed when the focus is deviated (defocused) in terms of focal depth. The changing rate of the focal point is expressed as Equation 3. EQU changing rate of focal point=tan .theta..sub.1 .apprxeq.sin .theta..sub.1 .varies.NA Equation 3
Finally, if the focal depth is set to .DELTA.Z, the focal depth .DELTA.Z is proportional to (f.sub.2 /f.sub.1) as shown in Equation 4. EQU .DELTA.Z .varies.f.sub.2 /f.sub.1 Equation 4
In the example in FIG. 8, it is proportional to (f.sub.2 /f.sub.1)=0.6. This value is a value of the laser plate making apparatus which is made into practical use by the present applicant, and it is sufficient to compensate for the aforesaid eccentricity and the like.
However, it is desirable to further improve the resolution in such laser plate making apparatus. A focal stripe width d.sub.H of approximately 70 .mu.m on the form plate sheet is desired in order to achieve a suitable improvement in resolution. Further, since the focal stripe width d.sub.v in the vertical direction is considerably small as compared with the focal stripe width d.sub.H in the horizontal direction (in the example in FIG. 8, stripe width d.sub.v is approximately 0.6 .mu.m), it is sufficient to consider only the focal stripe width d.sub.H in the horizontal direction.
In this way, in the example of the laser optical system in FIG. 8, when a necessary focal length f.sub.2 of the objective lens 4 is calculated under the condition that the focal stripe width d.sub.H =70 .mu.m, f.sub.2 =f.sub.1 .multidot.(d.sub.H /d.sub.LDH)=11.67.times.(70/200).apprxeq.4.08 mm is obtained from the above Equation 1.
However, if the above value is employed, the focal depth .DELTA.Z becomes .DELTA.Z.varies.0.35 from Equation 4, which is about a half the value as compared with that in the aforesaid prior art, resulting in a laser head having a shallow focal depth. Therefore, the above-described errors such as eccentricity of the plate cylinder, thickness of the form plate sheet or the like cannot adequately compensated for, and hence high resolution is difficult to achieve. Further, it is substantially difficult to reduce eccentricity and other errors up to about half or below as compared with that of the prior art by arranging other portions of the laser plate making apparatus due to high cost.
As such, an object of the present invention is to provide a laser plate making apparatus having a laser head wherein the focal depth is substantially maintained if the focal stripe width is made relatively small.